The transmission capacity of single-core optical fibers that are currently used is approaching its limit due to the rapidly increasing volume of optical communication traffic. In response to this situation, a multicore fiber in which multiple cores are formed in one fiber has been proposed as a means for increasing the communication capacity.
One example of such a multicore fiber has a plurality of core portions formed inside a cladding portion, and a part of the circumference of the cladding portion has a flat portion that is perpendicular to a longitudinal direction (Patent Document 1).
To use a multicore fiber as a transmission path, it is necessary to connect each core portion of the multicore fiber to a corresponding core portion of another multicore fiber, a different optical fiber, an optical device, or the like in order to send and receive transmission signals. A method of connecting such a multicore fiber to a single core fiber has been proposed in which the multicore fiber is connected to a fiber bundle including single-core optical fibers arranged at positions corresponding to core portions of the multicore fiber in order to send and receive transmission signals (Patent Document 2). A method of producing such an optical fiber bundle is also proposed in which a plurality of single-core fibers is bundled together by bonding or the like at predetermined distances (Patent Document 3).